Efforts to improve upon the performance of natural mineral oil based lubricants by the synthesis of oligomeric hydrocarbon fluids have been the subject of important research and development in the petroleum industry for at least fifty years and have led to the relatively recent market introduction of a number of synthetic lubricants. In terms of lubricant property improvement, the thrust of the industrial research effort on synthetic lubricants has been toward fluids exhibiting useful viscosities over a wide range of temperature, i.e., improved viscosity index, while also showing lubricity, thermal and oxidative stability and pour point equal to or better than mineral oil.
The viscosity-temperature relationship of a lubricating oil is one of the critical criteria which must be considered when selecting a lubricant for a particular application. The mineral oils commonly used as a base for single and multigraded lubricants exhibit a relatively large change in viscosity with a change in temperature. Fluids exhibiting such a relatively large change in viscosity with temperature are said to have a low viscosity index. Viscosity Index (VI) is an empirical number which indicates the rate of change in the viscosity of an oil within a given temperature range. A high VI oil, for example, will thin out at elevated temperatures slower than a low VI oil. The advantage of VI rating is that it capsulizes the effects of temperature as a single number. The viscosity index of a common paraffinic mineral oil is usually given a value of about 100. Viscosity index is determined according to ASTM Method D 2270-93 [1998] wherein the VI is related to kinematic viscosities measured at 40° C. and 100° C. using ASTM Method D 445-01. Both methods are fully incorporated by reference.
The American Petroleum Institute defines five groups of base stocks. Groups I, II and III are mineral oils classified by the amount of saturates and sulfur they contain and by their viscosity indices. Group I base stocks are solvent refined mineral oils. They contain less saturates and more sulfur and have lower viscosity indices. They define the bottom tier of lubricant performance. Group I stocks are the least expensive to produce, and they currently account for about 75 percent of all base stocks. These comprise the bulk of the “conventional” base stocks.
Groups II and III are hydroprocessed mineral oils. The Group III oils have higher viscosity indices than Group II oils do. Groups II and III stocks perform better thermal and oxidative stability. Isodewaxed oils also belong to Groups II and III. Isodewaxing rids these mineral oils of a significant portion of their waxes, which improves their cold temperature performance greatly. Groups II and III stocks account for about 20 percent of all base stocks.
Base Oil Group% Saturates% AromaticsVI% SulfurI<90>10<120>0.03II>90<10>80, <120<0.03III>90<10>120<0.03
Group II stocks may be “conventional” or “unconventional.” Generally, “unconventional” base stocks are mineral oils with unusually high viscosity indices and unusually low volatilities. Low severity hydroprocessing and solvent refined Group II mineral base stocks are “conventional.” Compared to Group I oils, severity hydroprocessed Group II and III oils offer lower volatility, and when properly additized, greater thermal and oxidative stability and lower pour points.
Group IV consists of polyalphaolefins. Group IV base stocks offer superior volatility, thermal stability, oxidative stability and pour point characteristics to those of the Group II and III oils with less reliance on additives. Currently, Group IV stocks, the PAOs, make up about 3 percent of the base oil market. Group V includes all other base stocks not included in Groups I, II, III and IV. Esters are Group V base stocks.
Polyalphaolefins (“PAOs”) comprise a class of hydrocarbons manufactured by the catalytic oligomerization (polymerization to low-molecular-weight products) of linear α-olefins typically ranging from 1-octene to 1-dodecene, with 1-decene being a preferred material, although polymers of lower olefins such as ethylene and propylene may also be used, including copolymers of ethylene with higher olefins, as described in U.S. Pat. No. 4,956,122 and the patents referred to therein. PAO products have achieved importance in the lubricating oil market.
The PAO products typically produced may be obtained with a wide range of viscosities varying from highly mobile fluids of low-viscosity, about 2 cSt., at 100° C. to higher molecular weight, viscous materials which have viscosities exceeding 100 cSt. at 100° C. PAOs are commonly classified according to their approximate kinematic viscosity (KV) at 100° C. The kinematic viscosity of a liquid is determined by measuring the time for a volume of the liquid to flow a given distance under gravity. Dynamic viscosity can then be obtained by multiplying the measured kinematic viscosity by the density of the liquid. The units for kinematic viscosity are 1 m2/s, commonly converted to cSt. or centistokes (IcSt.=10−6m2/s) with 1 cSt. being the viscosity of water at 20° C.
PAOs may be produced by the polymerization of olefin feed in the presence of a catalyst such as AlCl3, BF3, or BF3 complexes. Processes for the production of PAOs are disclosed, for example, in the following patents: U.S. Pat. Nos. 4,149,178; 3,382,291; 3,742,082; 3,769,363; 3,780,128; 4,172,855 and 4,956,122, which are fully incorporated by reference. PAOs are also discussed in Lubrication Fundamentals, J.G. PAO Wills, Marcel Dekker Inc., (New York, 1980). Subsequent to polymerization, the PAO lubricant range products are hydrogenated in order to reduce the residual unsaturation. In the course of this reaction, the amount of the residual unsaturation is generally reduced by greater than 90%.
Hydrocarbons generally, and in particular synthetic PAOs, have found wide acceptability and commercial success in the lubricant field for their superiority to mineral based lubricants. In terms of lubricant property improvement, industrial research efforts on synthetic lubricants have led to PAO fluids exhibiting useful viscosities over a wide range of temperature, i.e., improved viscosity index, while also showing lubricity, thermal and oxidative stability and pour point equal to or better than mineral oil. These relatively new synthetic lubricants lower mechanical friction, enhancing mechanical efficiency over the full spectrum of mechanical loads and do so over a wider range of operating conditions than mineral oil.
In accordance with customary practice in the lubricant arts, PAOs have been blended with a variety of additives such as functional chemicals, oligomers and polymers and other synthetic and mineral oil based lubricants to confer or improve upon lubricant properties necessary for applications, such as engine lubricants, hydraulic fluids, gear lubricants, etc. Blends and their additive components are described in Kirk-Othmer Encyclopedia of Chemical Technology, fourth edition, volume 15, pages 463-517, which is fully incorporated by reference.
A particular goal in the formulation of blends is the enhancement of viscosity index by the addition of VI improvers which are typically high molecular weight synthetic organic molecules. Such additives are commonly produced from polyisobutylenes, polymethacrylates and polyalkylstyrenes, and used in the molecular weight range of about 45,000 to about 1,700,000. While effective in improving viscosity index, these VI improvers have been found to be deficient because the very property of high molecular weight that makes them useful as VI improvers also confers upon the blend a vulnerability in shear stability during actual applications. Temporary shear results from the non-Newtonian viscometrics associated with solutions of high molecular weight polymers and is caused by an alignment of the polymer chains with the shear field under high shear rates with a resultant decrease in viscosity. The decreased viscosity reduces the wear protection associated with viscous oils. Newtonian fluids, in contrast, maintain their viscosity regardless of shear rate. This deficiency in shear stability dramatically reduces the range of useful applications for many VI improver additives. Accordingly, workers in the lubricant arts continue to search for better lubricant blends with high viscosity indices.
Current market conditions are extremely favorable for lubricant compositions which provide lower operating temperatures, increased operating efficiency, and increased hardware durability. With the advent of longer axle and transmission oil change intervals (ca 250,000 to 500,000 miles), durability is clearly at issue as well. Accordingly, the present invention meets these needs by allowing for the preparation of multigraded automotive gear lubricants, and lubricating fluids, which out perform prior art formulations and have none, or a greatly decreased amount of, the deficiencies found in the currently commercially available lubricants.